These brakes are air assisted and have a valve that gets Air pressure from the exhaust via a valve that has to release to let the brake fluid back into the reservoir. That valve can stick one way and restrict the release of the calipers as fluid can't get back to reservoir. It's a probable recall listed problem that you need to check by entering the VIN into a service system by Workhorse. You can force it back by lifting on the brake peddle with the top of your foot. That forces the valve to open and may solve the problem, which may reoccur if unit sits for a while. You also should check for an exhaust leak or a head gasket leak on the side the brake reservoir is, as the steam from a leaky head gasket will cause the valve to malfunction. That head gasket leak can be determined by a compression test. The lowering of Antifreeze and a sweet smell it gives off when it is burnt in the exhaust pipe is sometime present to suggest the head gasket leak. Check your specific VIN for symptoms and resolutions particular to that unit.
